I can't honestly say I've given thought to this, but knowing I've maintained my youthfulness better than most I know, I'd elderly, will arrive when I turn grey, need a cane for walking, and can no longer see without my spectacles.
As for "when" do I describe someone as being elderly? When I see an older person hunched over, walking slowly, with reduced mobility and having moderate trouble with simple motor skills such as opening a door, writing, or with tasks in and around the home, then I would think of them as being elderly, otherwise I always use the term, mature.